// SPDX-License-Identifier: GPL-2.0
/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
 * generic spinlock implementation
 *
 * Copyright (c) 2001   David Howells (dhowells@redhat.com).
 * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
 * - Derived also from comments by Linus
 */
#include <seminix/rwsem.h>
#include <seminix/sched/signal.h>
#include <seminix/sched/task.h>

enum rwsem_waiter_type {
    RWSEM_WAITING_FOR_WRITE,
    RWSEM_WAITING_FOR_READ
};

struct rwsem_waiter {
    struct list_head list;
    struct task_struct *task;
    enum rwsem_waiter_type type;
};

int rwsem_is_locked(struct rw_semaphore *sem)
{
    int ret = 1;
    unsigned long flags;

    if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
        ret = (sem->count != 0);
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    }
    return ret;
}

/*
 * initialise the semaphore
 */
void init_rwsem(struct rw_semaphore *sem)
{
    sem->count = 0;
    raw_spin_lock_init(&sem->wait_lock);
    INIT_LIST_HEAD(&sem->wait_list);
}

/*
 * handle the lock release when processes blocked on it that can now run
 * - if we come here, then:
 *   - the 'active count' _reached_ zero
 *   - the 'waiting count' is non-zero
 * - the spinlock must be held by the caller
 * - woken process blocks are discarded from the list after having task zeroed
 * - writers are only woken if wakewrite is non-zero
 */
static inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
{
    struct rwsem_waiter *waiter;
    struct task_struct *tsk;
    int woken;

    waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);

    if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
        if (wakewrite)
            /* Wake up a writer. Note that we do not grant it the
             * lock - it will have to acquire it when it runs. */
            wake_up_process(waiter->task);
        goto out;
    }

    /* grant an infinite number of read locks to the front of the queue */
    woken = 0;
    do {
        struct list_head *next = waiter->list.next;

        list_del(&waiter->list);
        tsk = waiter->task;
        /*
         * Make sure we do not wakeup the next reader before
         * setting the nil condition to grant the next reader;
         * otherwise we could miss the wakeup on the other
         * side and end up sleeping again. See the pairing
         * in rwsem_down_read_failed().
         */
        smp_mb();
        waiter->task = NULL;
        wake_up_process(tsk);
        put_task_struct(tsk);
        woken++;
        if (next == &sem->wait_list)
            break;
        waiter = list_entry(next, struct rwsem_waiter, list);
    } while (waiter->type != RWSEM_WAITING_FOR_WRITE);

    sem->count += woken;

 out:
    return sem;
}

/*
 * wake a single writer
 */
static inline struct rw_semaphore *
__rwsem_wake_one_writer(struct rw_semaphore *sem)
{
    struct rwsem_waiter *waiter;

    waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
    wake_up_process(waiter->task);

    return sem;
}

/*
 * get a read lock on the semaphore
 */
int __sched __down_read_common(struct rw_semaphore *sem, int state)
{
    struct rwsem_waiter waiter;
    unsigned long flags;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    if (sem->count >= 0 && list_empty(&sem->wait_list)) {
        /* granted */
        sem->count++;
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        goto out;
    }

    /* set up my own style of waitqueue */
    waiter.task = current;
    waiter.type = RWSEM_WAITING_FOR_READ;
    get_task_struct(current);

    list_add_tail(&waiter.list, &sem->wait_list);

    /* wait to be given the lock */
    for (;;) {
        if (!waiter.task)
            break;
        if (signal_pending_state(state, current))
            goto out_nolock;
        set_current_state(state);
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        schedule();
        raw_spin_lock_irqsave(&sem->wait_lock, flags);
    }

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 out:
    return 0;

out_nolock:
    /*
     * We didn't take the lock, so that there is a writer, which
     * is owner or the first waiter of the sem. If it's a waiter,
     * it will be woken by current owner. Not need to wake anybody.
     */
    list_del(&waiter.list);
    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    return -EINTR;
}

void __sched __down_read(struct rw_semaphore *sem)
{
    __down_read_common(sem, TASK_UNINTERRUPTIBLE);
}

int __sched __down_read_killable(struct rw_semaphore *sem)
{
    return __down_read_common(sem, TASK_KILLABLE);
}

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
int __down_read_trylock(struct rw_semaphore *sem)
{
    unsigned long flags;
    int ret = 0;


    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    if (sem->count >= 0 && list_empty(&sem->wait_list)) {
        /* granted */
        sem->count++;
        ret = 1;
    }

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

    return ret;
}

/*
 * get a write lock on the semaphore
 */
int __sched __down_write_common(struct rw_semaphore *sem, int state)
{
    struct rwsem_waiter waiter;
    unsigned long flags;
    int ret = 0;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    /* set up my own style of waitqueue */
    waiter.task = current;
    waiter.type = RWSEM_WAITING_FOR_WRITE;
    list_add_tail(&waiter.list, &sem->wait_list);

    /* wait for someone to release the lock */
    for (;;) {
        /*
         * That is the key to support write lock stealing: allows the
         * task already on CPU to get the lock soon rather than put
         * itself into sleep and waiting for system woke it or someone
         * else in the head of the wait list up.
         */
        if (sem->count == 0)
            break;
        if (signal_pending_state(state, current))
            goto out_nolock;

        set_current_state(state);
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        schedule();
        raw_spin_lock_irqsave(&sem->wait_lock, flags);
    }
    /* got the lock */
    sem->count = -1;
    list_del(&waiter.list);

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

    return ret;

out_nolock:
    list_del(&waiter.list);
    if (!list_empty(&sem->wait_list) && sem->count >= 0)
        __rwsem_do_wake(sem, 0);
    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

    return -EINTR;
}

void __sched __down_write(struct rw_semaphore *sem)
{
    __down_write_common(sem, TASK_UNINTERRUPTIBLE);
}

int __sched __down_write_killable(struct rw_semaphore *sem)
{
    return __down_write_common(sem, TASK_KILLABLE);
}

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
int __down_write_trylock(struct rw_semaphore *sem)
{
    unsigned long flags;
    int ret = 0;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    if (sem->count == 0) {
        /* got the lock */
        sem->count = -1;
        ret = 1;
    }

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

    return ret;
}

/*
 * release a read lock on the semaphore
 */
void __up_read(struct rw_semaphore *sem)
{
    unsigned long flags;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    if (--sem->count == 0 && !list_empty(&sem->wait_list))
        sem = __rwsem_wake_one_writer(sem);

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}

/*
 * release a write lock on the semaphore
 */
void __up_write(struct rw_semaphore *sem)
{
    unsigned long flags;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    sem->count = 0;
    if (!list_empty(&sem->wait_list))
        sem = __rwsem_do_wake(sem, 1);

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}

/*
 * downgrade a write lock into a read lock
 * - just wake up any readers at the front of the queue
 */
void __downgrade_write(struct rw_semaphore *sem)
{
    unsigned long flags;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    sem->count = 1;
    if (!list_empty(&sem->wait_list))
        sem = __rwsem_do_wake(sem, 0);

    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
}

